1. Field of the Invention
The present invention relates to control systems and, more particularly, to control of rectifiers employing semiconductor devices, such as silicon controlled rectifiers (SCRs) or thyristors. The invention also relates to a method for controlling a rectifier bridge.
2. Background Information
Three-phase rectifier circuits are commonly employed to convert AC signals to DC signals. These circuits often use SCRs disposed in bridge segments, with typically one SCR for each polarity of each AC phase. Typically, a bridge firing control circuit controls the firing point for each rectifier in each AC cycle. Examples of such circuits arc disclosed in U.S. Pat. Nos. 5,963,440; 5,963,441; 6,046,917; 6,208,120; and 6,232,751.
It is not uncommon for a plurality of SCR bridges to be operated in parallel with each of the corresponding bridge firing control circuits being controlled by a central firing control circuit. The central firing control circuit manages each of the bridge firing control circuits in order that the corresponding rectifiers in each of the parallel bridges conduct current at the same point in the AC waveform.
SCR bridges are commonly employed in an excitation control system to provide field excitation for a rotating electrical apparatus (e.g., large synchronous generators and motors, utility synchronous generators and motors, industrial synchronous motors and generators, synchronous generators and motors for naval or other shipping applications, synchronous generators and motors for oil well drilling rigs).
A bridge converter may include two or more parallel bridges. Each one of these bridges is configured in parallel with the other one or more bridges, in order that they share the load current.
U.S. Pat. No. 5,963,441 discloses a xe2x80x9cskip firingxe2x80x9d SCR control method, which is employed to control the current balance in parallel devices to obtain the desired effect. The basic principle is to control the average current of an element by xe2x80x9cnot firingxe2x80x9d the element a controlled number of times or by varying the point at which the element fires. Because of the large inductance of the load on parallel bridges, establishing or controlling appropriate duty cycles for the individual bridges by skipping cycles or adjusting the cycles does not introduce appreciable disturbance in the load current. In turn, skip firing can be used to control the average current balance in parallel cells or can be used to unbalance a system to compensate for an element that is heating up.
Bridge firing control (BFC) circuits, which communicate with a central firing control circuit or exciter firing control (EFC), determine the average current for each cell and send those results to the EFC. Sensors, such as resistance temperature devices, may be employed to sense heat sink temperatures for the individual bridge segments associated with each, SCR. Those temperatures may be recorded and, then, sent to the EFC. Algorithms in the EFC calculate when and how often each cell in each bridge should not be fired or have its firing period advanced or retarded. In turn, the EFC sends this information (i.e., the xe2x80x9cskip firing codexe2x80x9d) over a communication bus to the BFCs. Each of the BFCs then skip fires based on the skip firing code (e.g., how many cycles to skip, how those cycles are to be distributed over a time interval, and how those skips are timed so as not to skip simultaneously the corresponding segment on any other bridge). The process continues with the collection of more data and subsequent modification to the skip firing code by the EFC.
U.S. Pat. No. 5,963,440 discloses employing skip firing to achieve current balance between cells. Over a given time interval, bridge firing control (BFC) circuits measure the current in each cell in the bridges. At the end of a time interval, the average current for each cell is calculated and sent to the central firing control circuit or EFC. Heat sink temperatures are also recorded from the sensors or resistance temperature devices. The recorded information is sent to the EFC. Algorithms in the EFC calculate when and how often each cell in each bridge should not be fired or have its firing period advanced or retarded. The EFC sends this information, the xe2x80x9cskip firing code,xe2x80x9d over a communication bus to the BFC circuits, each of which then skip fires based on the xe2x80x9cskip firing codexe2x80x9d. The process continues with the collection of more data and subsequent modification to the xe2x80x9cskip firing codexe2x80x9d by the EFC.
The xe2x80x9cskip firing codexe2x80x9d is a code designed to be sent to each BFC by the EFC over the communication bus. The code is designed to tell each bridge how many cycles to skip and how those cycles are to be distributed over a time interval.
There is room for improvement in control systems and methods for controlling rectifier bridges.
This need and others are met by the present invention, which employs an active temperature balance algorithm for controlling signals to elements or semiconductor devices, such as a thyristors or SCRs, of parallel rectifier bridges, in order to balance the temperatures of the parallel elements. This is accomplished by averaging sensed temperatures to provide a corresponding average (or xe2x80x9cfair sharexe2x80x9d) temperature for each of the parallel elements of the rectifier bridges, comparing one of the input sensed temperatures to the corresponding average temperature, skipping repetitive firing of at least one of the elements for one out of a plurality of counts, increasing the counts when the sensed temperature of one of the at least one of the elements is less than the corresponding average temperature or decreasing the counts when the sensed temperature of the one of the at least one of the elements is greater than the corresponding average temperature, and setting the counts to a first predetermined value when the input sensed temperature of the one of the at least one of the elements is greater than a second predetermined value above the corresponding average temperature.
As one aspect of the invention, a control system comprises: a firing control circuit outputting a plurality of firing commands; a parallel array of a plurality of rectifier bridges, each of the: rectifier bridges converting a plurality of alternating current voltages from a plurality of alternating current phases to a voltage, each of the rectifier bridges comprising a plurality of segments, each of the segments having an element, each of the elements of one of the rectifier bridges having a temperature and being electrically interconnected in parallel with at least a corresponding one of the elements of the other of the rectifier bridges; a plurality of temperature sensors, each of the temperature sensors sensing the temperature of a corresponding one of the elements; a plurality of bridge control circuits, each of the bridge control circuits inputting one of the firing commands, outputting a plurality of control signals responsive to the one of the firing commands to repetitively fire at least some of the elements of a corresponding one of the rectifier bridges, and inputting the sensed temperatures of the elements of the corresponding one of the rectifier bridges; a communication channel communicating the input sensed temperatures from the bridge control circuits to the firing control circuit, and communicating the firing commands from the firing control circuit to the bridge control circuits; and an output having the voltage, wherein the firing control circuit includes means for averaging some of the input sensed temperatures to provide a corresponding average temperature for each of the parallel elements of the rectifier bridges, for comparing one of the input sensed temperatures to the corresponding average temperature, for skipping repetitive firing of at least one of the elements for one out of a plurality of counts, for increasing the counts when the input sensed temperature of one of the at least one of the elements is less than the corresponding average temperature or for decreasing the counts when the input sensed temperature of the one of the at least one of the elements is greater than the corresponding average temperature, and for setting the counts to a first predetermined value when the input sensed temperature of the one of the at least one of the elements is greater than a second predetermined value above the corresponding average temperature.
The means for averaging may include a processor and a routine setting the counts to the first predetermined value when a count of the counts is greater than the first predetermined value. The means for averaging may include a processor and a routine to compare each of the input sensed temperatures to a threshold temperature and to responsively enable the increasing or the decreasing whenever any of the input sensed temperatures exceeds the threshold temperature.
The means for averaging may include a processor and a routine to increment the counts by one when the input sensed temperature of the one of the at least one of the elements is less than the corresponding average temperature. The means for averaging may include a processor and a routine to decrement the counts by one when the input sensed temperature of the one of the at least one of the elements is greater than the corresponding average temperature.
The routine may disable repetitive firing of the at least one of the elements when a count of the counts is a maximum value.
As another aspect of the invention, a method controls a parallel array of a plurality of rectifier bridges, each of which converts a plurality of alternating current voltages from a plurality of alternating current phases to a voltage, and each of which comprises a plurality of segments, each of the segments has an element, each of the elements of one of the rectifier bridges has a temperature, is electrically interconnected in parallel with at least a corresponding one of the elements of the other of the rectifier bridges, and is responsive to a control signal. The method comprises: outputting the control signals to repetitively fire the elements; skipping repetitive firing of at least one of the elements for one out of a plurality of counts; sensing the temperatures of the elements; averaging some of the sensed temperatures to provide a corresponding average temperature for each of the parallel elements of the rectifier bridges; comparing one of the sensed temperatures to the corresponding average temperature; increasing the counts when the sensed temperature of one of the at least one of the elements is less than the corresponding average temperature, or decreasing the counts when the sensed temperature of the one of the at least one of the elements is greater than the, corresponding average temperature; and setting the counts to a first predetermined value when the sensed temperature of the one of the at least one of the elements is greater than a second predetermined value above the corresponding average temperature.
The method may further comprise setting the counts to the first predetermined value when a count of the counts is greater than the first predetermined value.
The method may compare each of the sensed temperatures to a threshold temperature and conditionally start the decreasing or the increasing whenever any of the sensed temperatures exceeds the threshold temperature. The method may disable the skipping until after any of the sensed temperatures exceeds the threshold temperature.
The method may further comprise incrementing the counts by one when the sensed temperature of the one of the at least one of the elements is less than the corresponding average temperature, or decrementing the counts by one when the sensed temperature of the one of the at least one of the elements is greater than the corresponding average temperature.
The method may enable the skipping after any of the sensed temperatures exceeds the threshold temperature and independent of any subsequent sensed temperatures.
As another aspect of the invention, the method comprises: outputting the control signals to repetitively fire the elements; skipping repetitive firing of at least one of the elements at a skip frequency; sensing the temperatures of the elements; averaging some of the sensed temperatures to provide a corresponding average temperature for each of the parallel elements of the rectifier bridges; comparing one of the sensed temperatures to the corresponding average temperature; decreasing the skip frequency when the sensed temperature of one of the at least one of the elements is less than the corresponding average temperature, or increasing the skip frequency when the sensed temperature of the one of the at least one of the elements is greater than the corresponding average temperature; and setting the skip frequency to a predetermined skip frequency value when the sensed temperature of the one of the at least one of the elements is greater than a predetermined value above the corresponding average temperature.
The method may further comprise setting the skip frequency to the predetermined skip frequency value when the skip frequency is less than the predetermined skip frequency value.